Introduction:
The process of electricity generation from fossil fuels such as coal, oil and natural gas is water intensive. Between 40 -50% of all water abstracted and used in developed countries is used in the generation generation of electricity. Water supplies are required to provide various process waters for the following essential main purposes: Boiler makeup water to the water/steam circuit 2. Cooling water for steam turbine condenser Auxiliary plant cooling water Auxiliary Makeup water to flue gas desulphurization (FGD) plant (where fitted) Ash handling and disposal (coal fired power power plant only) Ash 6. General domestic use
If suspended solids in the water supply are high and likely to lead to silting and fouling problems in cooling towers and cooling water circuits, clarification and/or filtration of the water may be necessary before use. Water used to supply makeup water treatment plants will generally require some form of clarification and/or filtration prior to its treatment by ion exchange or membrane technology. Colloidal Species Colloidal silica is non-ionic and cannot be removed by ion exchange technology. Partial removal of colloids can be achieved by flocculation/clarification. Complete removal can only be achieved by use of appropriate membrane separation technology Power Plant Water Demand
Impurities in process water: Total Dissolved Solids For many power plant process water requirements using freshwater supplies, it is unnecessary to remove or reduce the total dissolved solids prior to use. However, in some particular processes where the process water is subject to substantial evaporation losses, this results in a concentration of the dissolved solids in the process water, e.g. in a recirculating cooling tower system. As high concentrations of dissolved solids in the cooling water may cause scale formation by precipitation of calcium salts, it may be necessary to mitigate potential scaling problems by controlling the concentration of these salts through system purging and/or use of anti-scalant chemical additives. In the production of makeup water for modern high pressure boiler plant only traces of ionic impurities in the makeup water are permitted. It is therefore necessary to remove all ionic constituents from the raw water to achieve the high purity that is required. Ion exchange processes were the most common treatment technology used to provide high purity makeup water to high pressure boilers. However, in the past few years, due to proven reliability and performance, membrane separation technology has become more acceptable Organic Matter The presence of dissolved organic matter in boiler makeup water supplies can pose serious potential risks to the integrity of steam turbines. Within the steam/water circuit of boiler plant these dissolved organic species undergo degradation to form simple organic acids species such as acetic and formic acids... These acidic species in steam tend to concentrate in the first condensate droplets/films formed within the turbine, depressing the pH at the dry/wet steam phase transition zone in the low pressure cylinder. Organic acids in steam have been known to cause general corrosion in turbine cylinders. Additionally, acetate and formate species have been linked to a specific role in stress corrosion cracking Dissolved organic species are only partially removed by conventional coagulation and clarification. Anion exchange resins have the capability to remove dissolved organics but their efficacy is dependent on the size, polarity and concentration of the organic molecules present in the water. Additionally, anion exchange resins are susceptible to fouling by dissolved organic species which has a detrimental impact on their performance. The only treatment technology currently available that will consistently reduce organic levels in makeup water supplies to acceptable levels is reverse osmosis membrane technology. The larger suspended organic material present in surface waters can often be removed by coarse screening or conventional filtration. Finer particulate would require a combination of clarification and filtration. Suspended solids:
The largest single demand for water has b een associated with the cooling water system for the steam turbine condenser. The function of this cooling water system is to cool and condense the steam exhausting from the turbine to enable its return to the boiler water/steam circuit as feedwater. In condensing the steam, the temperature of the cooling water typically increase by 5 – 10oC as it passes through the condenser absorbing the latent heat of the condensed steam. There are principally two types of wet cooling system employed in fossil fired power plant: once through/direct cooling system and open recirculating cooling system. In the once though cooling system, the cooling water is abstracted from the water source, pumped through the condenser and then the warm water is discharged immediately back to the source of abstraction. Once through cooling systems are mainly employed at coastal power plant where large volumes of water are normally available for cooling. At power plants located inland, rivers are unable to provide the large volumes of water required by once through cooling systems. Hence, inland plants typically utilise the open recirculating cooling system with evaporative cooling towers to reduce their cooling water requirements. As a comparison, the cooling water abstraction requirement for a typical 2000MW coal-fired station with a once through system is approximately 6.5 million m3/day. In contrast, a similar power plant with a recirculating cooling system would only require 0.2 million m3/day. The second largest single consumer of water is the makeup water treatment plant. The function of this plant is to produce water for use in the boilers of fossil fuel power plants. The makeup water replaces water and steam lost from the plant water/steam circuit as a result of leaks, drainage, boiler blowdown and any steam exported to third parties. The makeup water for modern fossil fuel boilers has to be high purity deionised water with virtually complete removal of impurities. The quantity of boiler makeup water required
generally ranges from 1 –3% of the maximum continuous rated steam flow.
or raw water treatment. However, not all wastewaters will be viable options for recovery and reuse.
Pressures for Water Reuse Various factors are contributing to a greater interest in water conservation within power plant These include: Increasing pressures on water resources; Compromised quality of water sources; Increasingly stringent environmental legislation regarding water use; Increasing costs of water abstraction a nd discharge.
At power plant worldwide, reuse schemes can extend from the recovery of individual high quality waste streams, which can be reused either directly or after only limited treatment, though to the development of fully integrated water/wastewater treatment systems for zero liquid discharge (ZLD). Integrated water/wastewater treatment systems link together the cooling tower, demineraliser system and the ZLD equipment in order to minimise site water usage and maximise water recycling. Zero liquid discharge (ZLD) plant designs are becoming more common place in the power industry, driven by environmental restrictions on discharges and limitations on water supplies. ZLD can also be achieved by water resource sharing, where power plant wastewaters can be reused in applications offsite, rather than discharging directly to waste.
These pressures are expected to have an increasing influence on water treatment decisions. In general, pressures on water availability and water quality have typically been limited to plant located in arid or industrialized regions or at locations remote from viable surface or ground water sources to date.
Opportunities for Water Reuse
In emerging economies in particular, industrial plants have had difficulties with both availability and quality of raw water supplies. Heavy pollution is prevalent in many Asian countries and lack of adequate municipal water treatment plant means that industrial users will tend to receive poorer quality source water. In order to reduce reliance on potable water supplies, increased use of alternative water sources at power plant is expected. Industrial wastewaters, brackish water, poor quality groundwater and seawater are all now viable options for treatment. The use of reclaimed municipal wastewater in power plant has seen particular growth over the last decade as a result of increasing limitations on water supplies.
In this section, the potential for wastewater reuse in the main power plant water consuming processes is discussed. The most common power plant wastewater streams recovered for reuse are boiler blowdown waters, demineralisation plant effluents, ash handling wastewaters and cooling tower blowdown.
Growing pressures on water availability and measures required to control industrial pollution have resulted in environmental control legislation becoming increasingly stringent. For power plant, pressures on water availability are making access to water more difficult and more expensive.
Various process wastewaters can be utilised as make up to replace evaporative losses from wet cooling systems. The suitability of a wastewater for reuse in cooling water systems is dependent largely on the characteristics of the effluent stream, particularly the tendency of the wastewater for scaling, fouling and corrosion. The tendency for corrosion must be considered to ensure compatibility with system metallurgy.
Consequently, measures are increasingly being implemented by power plant to reduce both the amount of water abstracted for plant use and the amount of wastewater discharged. This is being achieved in a number of ways, including the optimization of water use within processes and the application of water recovery and reuse schemes, particularly in new build plant. The technical and economic feasibility of wastewater reuse schemes at power plant requires site specific consideration and will be dependent on the particular drivers and policy with regards to water use at any given location. Relevant factors include: the quality, quantity and cost of raw water available to the plant; the quality and quantity of water needed for various plant processes; the ability to recycle wastewater streams to other plant processes, either directly or after suitable treatment; the wastewater treatment technologies available; capital, operational, maintenance and labour costs and floor space and construction requirements; environmental restrictions on the quantity and quality of any wastewater that may be discharged. In general, the application of recycling normally requires some suitable process technology for water purification, or temperature reduction for the recovery of hot boiler waters. The treatment process necessary will be dependent on the quality and quantity of the wastewater and the purity requirements of the recipient process. The treatment costs incurred to enable water reuse may be offset by savings from reduced water abstraction, discharge,
Whilst these waste streams are all produced internally by power plant processes, water reuse can also be considered as the reuse of effluent produced by external industrial processes. Cooling Water Systems
The design and operation of cooling water systems may not permit the reuse of some waters.
Demineralised Water Treatment Plant High quality wastewaters, such as boiler blowdown and condensates, can be recycled to the demineralised water treatment plant for boiler make up water production. These types of water are often of a higher quality than the demineralisation plant raw water supply and reuse can present cost savings due to both reduced raw water abstraction and reduced treatment requirements. Temperature reduction of boiler blowdown is required before reuse is practicable. In modern power plant, this is achieved generally by mixing the blowdown with quench water. As the quench water employed is often from a town’s main supply or sea water, this impacts upon the quality of the final blowdown stream. Flue Gas Desulphurisation Processes Wet flue gas desulphurisation (FGD) systems are major water consumers in the water balance of coal-fired plant. Most water is lost through evaporation with the exiting flue gas, whilst a smaller volume is lost with the dewatered solids. FGD equipment is already designed to handle liquid streams that are high in dissolved salt content and precipitated gypsum solids. Therefore, low quality waters can generally be used as make up water for FGD systems without affecting plant performance. The make
up water for FGD plant is generally abstracted from the cooling water system. Depending on quality, make up water can be introduced into the FGD system in a variety of areas, including mist eliminator wash, reagent preparation and feed system, gypsum wash water and pump and equipment seal flushing. However, each of these areas may require different levels of water quality with respect to dissolved and suspended solids. In the limestone gypsum process, careful consideration needs to be given to the effects of recycling wastewaters containing high concentrations of chloride salts on purge flows from the absorber, and to the potential impact of impurities contained in the make up water on produced gypsum quality. Coal and Ash Handling Coal and ash handling processes can tolerate low quality wastewaters containing relatively high levels of suspended and dissolved solids. These processes require large quantities of water for dust suppression and equipment washes. Substantial volumes of water can also be required for ash sluice systems and fly ash transport, although this is dependent on the specific waste handling process employed. Wastewater from furnace bottom ash transport can also be reduced by the use of a closed-loop recirculating system. Equipment Washes Raw water consumption can be reduced by using low quality wastewaters for maintenance and cleaning activities, such as air pre-heater washes and equipment washes in the FGD system and coal handling areas. Reuse of Industrial Wastewaters Various wastewaters from external industrial processes can be utilised as make up water for power plant water consuming processes if power plant and industry are located adjacently. Most such schemes involve power plant wet cooling tower systems, which are generally the largest water consuming processes at sites. For significant power plant dependence on i ndustrial wastewaters, this clearly requires long term guarantee of supply. Secondary or tertiary treated effluent from municipal sewage plants is a viable source for cooling tower make up due to the relatively large volumes obtainable. Wastewater from municipal sewage plants, known as either reclaimed water or grey water, has been used in power plant cooling systems for many years. Reclaimed waters characteristically contain high concentrations of organic matter, ammonia, phosphorous and bacterial growths. This has resulted in biological fouling, deposition, corrosion and scale formation in a number of cooling water applications where circuit chemistry has not been controlled adequately. Wastewater from power plant processes can also be reused externally in industrial processes. Examples in the literature include water discharged from power plant cooling water systems being used as feedwater for external industrial processes, desalination and potable water production, and in agricultural irrigation schemes. Blowdown from the cooling water system is then used as feedwater for a local alumina processing plant where there is no requirement for high purity water. Reduced and Zero Liquid Discharge Systems Introduction By definition, reduced or zero liquid discharge (ZLD) processes treat significant volumes of low quality wastewater, such that the waste stream is greatly reduced, or eliminated, and the bulk of the wastewater becomes reusable. The minimisation of blowdown from wet cooling tower systems is a
key part of any ZLD process, as this is generally the largest wastewater discharge at a facility. In ZLD applications, a number of different plant wastewater streams may be combined for processing. The initial stage in the treatment process usually involves volume reduction, as there are economic benefits in minimising the final waste stream for disposal. Reuse of the concentrated effluent is rarely practicable and a final treatment stage is usually required to convert the remaining dissolved solids to a suitable medium for disposal, most commonly to a solid product for landfill. The capital and operating costs of ZLD processes are usually substantial. ZLD applications frequently utilise a number of treatment stages and may require highly alloyed materials of construction for corrosion resistance, entailing high capital costs. Operating costs for ZLD technologies are often significant due to high energy consumption and chemical usage, and the disposal costs of waste sludges and solids. Processes that have been used for ZLD systems and cooling tower blowdown treatment, individually or in combination, include the following: Evaporation basins; Brine concentrators; Crystallisers; Membrane processes. Evaporation Ponds Evaporation ponds or basins can provide a simple, flexible and relatively low operating cost method for wastewater disposal. Ponds are relatively shallow to maximise surface area and designs typically include a system of liners with leak detection to protect ground water supplies. However, evaporation ponds are only suitable for plants located in arid regions, where solar evaporation significantly exceeds annual rainfall. Evaporation ponds are not practical options for most plant due to the climate at a given location or restrictions on available land space. Brine Evaporators/Concentrators
Brine concentrators have been used extensively for wastewater volume reduction at power plant, with over 100 installations used in zero liquid discharge systems. Brine concentrators are capable of recovering greater than 95% of a wastewater flow as high purity distillate (< 10ppm TDS). The distillate is generally reused as either cooling tower make up or as feedwater for demineralisation plant. The concentrated brine slurry produced (> 150,000ppm TDS) can be reduced to dry solids in a crystalliser or spray drier, or sent to an evaporation pond.
The brine concentration process utilises a vertical falling film evaporator with vapour recompression cycle and calcium sulphate seeding of the brine to control scaling Feedwater pre-treatment consists typically of filtration, acid injection and deaeration for the control of scaling and corrosion. The deaerated feedwater is fed to the evaporator sump for combination with circulating brine slurry. The brine slurry is continuously circulated from the sump to a flood box at the top of the heat transfer tubes, from where it is distributed and falls by gravity as a thin film down the inside of the tubes. As the brine falls back into the sump, a small portion is evaporated and drawn through mist eliminators to a compressor. The compressed vapour then flows over the outer tube surfaces, where heat is transferred to the cooler brine within the tubes. This causes a small amount of internally circulating brine to evaporate and condenses the external vapour as distilled water. The distillate is pumped back through a heat exchanger to pre-heat the incoming wastewater. To control evaporator slurry density, a small amount of the concentrated brine is continuously blown down for further processing or disposal.
mechanical vapour compression. Slurry from the volume reduction process is sent to the crystalliser sump and then to a flooded shell and tube heat exchanger. Because the tubes are flooded, the brine is under pressure and does not boil, thus preventing scaling. The brine enters the crystalliser vapour body at an angle and is swirled into a vortex. A small amount of the vapour evaporates and crystals form. Most of the brine is recirculated back to the heater. A small stream is removed for solids dewatering, usually using a filter press or centrifuge. The dewatered product is sent normally to a landfill site, but can be commercially saleable. Distillate recovered during the process is suitable for reuse in high quality water applications. Forced Circulation Steam-Driven Crystalliser The use of steam for heating simplifies the design and operation of the crystalliser, but does result in significant steam and cooling water requirements for a utility plant item. Vapour recompression crystallisers eliminate steam and cooling water requirements, but add electrical demand and are very susceptible to operational upsets, such as foaming. Crystallisers require highly alloyed materials, which again incurs a high capital cost.
Vapour Compression Brine Concentrator System Power plant turbine exhaust steam can also be used to evaporate wastewater in waste steam brine concentrators. These tap directly into the s team line between the power plant and its condenser, or into the condenser steam shell. By operating under vacuum and by using a proprietary configuration, wastewater can be evaporated at very low temperature (40 to 50 oC), greatly reducing the risk of corrosion and reducing energy requirements. Waste Steam Brine Concentrator
Spray Driers
The disadvantages of brine concentrators are largely economic, as they entail substantial capital and operating costs. High capital costs result from the use of titanium or stainless steel alloys for heat exchanger surfaces, which are necessary to tolerate the extremely corrosive conditions experienced when treating saline wastewaters. Brine concentrators consume large amounts of electricity, typically between 80 to 100kWh per 1000USgallons of wastewater treated, resulting in significant operating costs. To minimise the size of brine concentrator required, reverse osmosis is used frequently for wastewater volume reduction beforehand. Brine concentrators are reported to operate very reliably, particularly when run in steady state mode. Problems that have been experienced are usually the result of scale build up, which necessitates periodic outages for mechanical cleaning, high pressure water washing and chemical cleans. Stress corrosion cracking of stainless steel materials has been an issue at some plants and there are other examples in the literature of general corrosion Crystallisers Crystallisers are used to reduce the brine concentrate from volume reduction processes (such as brine concentrators and membrane processes) to a dry solid product. Crystallisers are thermal evaporators that can be driven by either steam or
Spray driers are alternatives to crystallisers for reducing concentrated brine solutions to solids for disposal. The spray drier consists of an atomising wheel spinning at approximately 17,000rpm that sprays the brine slurry into a hot, gas-filled chamber. Water instantly evaporates from the brine droplets and the solids are drawn into bag filters. Problems have been reported with the longevity of stainless steel wheels due to the corrosive and erosive nature of the brine slurry. A double-disk, ceramic lined, titanium wheel is reported to have worked well, with a lifetime of approximately three years Membrane Techniques Reverse osmosis (RO) has been used for wastewater volume reduction in a number of ZLD applications. RO is generally the least costly method of wastewater volume reduction and can
be used to concentrate wastewaters containing high levels of dissolved salts, silica and organic matter. Water recoveries of approximately 75% are quoted typically when used to treat power plant wastewaters. In reverse osmosis water reuse applications, the purified permeate water is of suitable quality for reuse in most plant areas. The concentrated reject stream is either processed further, for example i n a brine concentrator and/or crystalliser, or reused in low quality water applications. In wastewater treatment processes, RO membranes can be particularly susceptible to scaling and fouling and regular membrane cleaning can often be necessary. The key issue for successful RO plant operation is correct feedwater pre treatment, since wastewater streams can often be near saturation for several constituents and can also contain relatively high concentrations of suspended solids. RO pretreatment typically consists of lime softening and filtration in wastewater applications. However, membrane fouling has resulted frequently from poor softener/clarifier performance, often as a result of suspended solids carryover. Microfiltration (MF) and ultrafiltration (UF) are increasingly being used for the removal of suspended solids and biological matter from wastewaters before RO systems. High Efficiency Reverse Osmosis (HERO) has been used in several wastewater treatment applications. Membrane operation at high pH is reported to confer a number of advantages in comparison to convention RO systems that run at a near neutral or slightly acidic pH (6.5 – 7.0). These include greater rejection of weakly ionised species, higher permeate recovery rates (≥ 90%) and prevention of biological fouling. As with conventional RO, pre-treatment is critical for successful operation. Electrodialysis (EDI) and electrodialysis reversal (EDR) can be used in final polishing applications, but require a relatively high quality feedwater. Neither process can tolerate high levels of silica or organics. Of the two processes, EDI is the more commonly used at present.
